Wednesday, August 5, 2020

Ecosystem - Notes | Class 12 | Part 3: Energy Flow

14. ECOSYSTEM

3. ENERGY FLOW

Sun is the only source of energy for all ecosystems (except deep sea hydro-thermal ecosystem).

Of the incident solar radiation, less than 50% is photosynthetically active radiation (PAR).

Plants and photosynthetic bacteria (autotrophs), fix solar radiant energy to make food.

Plants capture only 2-10% of the PAR. This energy sustains the entire living world.

Ecosystems obey 2nd Law of thermodynamics. They need a constant supply of energy to synthesize the molecules. It helps to counteract the entropy.

Producers (Autotrophs):

These are organisms that synthesize food.

In a terrestrial ecosystem, major producers are herbaceous and woody plants. Primary producers in an aquatic ecosystem are phytoplankton, algae and higher plants.

The energy trapped by the producer is passed on to a consumer or the organism dies.

Consumers (heterotrophs):


These are animals that directly or indirectly depend on plants for food. They include:
  • Primary consumers (herbivores): Feed on plants. E.g. insects, birds, mammals, molluscs etc.
  • Secondary consumers (primary carnivores): Feed on herbivores. E.g. frog, fox, man etc.
  • Tertiary consumers (secondary carnivores): Feed on primary carnivores. E.g. tiger, lion etc.
The chain of feeding relationship between different organisms is called a food chain. It is 2 types:
  • Grazing Food Chain (GFC): Here, primary consumer feeds on living plants (producer). E.g.
  • Detritus Food Chain (DFC): Here, primary consumer feeds on dead organic matter (detritus). Death of organism is the beginning of the DFC.
Detritus is made up of decomposers (saprotrophs) such as fungi & bacteria. They secrete digestive enzymes that breakdown detritus into simple, inorganic materials, which are absorbed by them. Thus, they get energy & nutrients.

In an aquatic ecosystem, GFC is the major conduit for energy flow.

In a terrestrial ecosystem, a much amount of energy flows through the DFC than through the GFC.

DFC may be connected with GFC at some levels. Some organisms of DFC are prey to the GFC animals. Some animals (cockroaches, crows, human etc.) are omnivores. Such interconnections of food chains are called food web.

A specific place of organisms in the food chain is known as their trophic level.


The amount of energy decreases at successive trophic levels. When an organism dies it becomes dead biomass (detritus). It is an energy source for decomposers.

Organisms at each trophic level depend on those at the lower trophic level for their energy.

The amount of living material in a trophic level at a given time is called standing crop. It is measured as the biomass (mass of living organisms) or the number in a unit area.

Biomass of a species is measured in terms of fresh or dry weight. Dry weight is more accurate because it is the exact mass of body which remains constant.

Number of trophic levels in GFC is restricted as it follows 10% law (only 10% of energy is transferred to each trophic level from the lower trophic level).
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